Add Vulkan Forward+ PBR regression guards

[cursor]

Summary

• Add a deterministic Python CTest source guard for Vulkan Forward+ luminance-sort wiring and PBR fragment specialization-map bounds.
• Document the new validation target and the existing VM native-module unit in tests/README.md.

Risky behavior now covered

• PBR ADD_FRAG_SPEC entries must remain contiguous, within both PBR/non-PBR spec_entries[] capacities, and aligned with GLSL constant_id declarations.
• forward_plus_shade_strength stays mapped to constant id 40 and clamped before pipeline specialization.
• r_forwardPlusLuminanceSort remains default-on, range-checked, runtime-toggleable, pushed into the compute shader, and honored by shader-side brightest-light selection.

Test files added/updated

• Added tests/scripts/test_vulkan_forward_plus_pbr_guards.py.
• Updated CMakeLists.txt to register test_vulkan_forward_plus_pbr_guards when Python3 is available.
• Updated tests/README.md test inventory.

Why this reduces regression risk

These paths are high-blast-radius renderer logic that is difficult to exercise on headless CI with real Vulkan hardware. The new guard catches source-level regressions that could otherwise reintroduce specialization-map overflows, shader/C constant-id drift, or broken Forward+ luminance sorting.

Validation

• python3 tests/scripts/test_vulkan_forward_plus_pbr_guards.py /workspace
• cmake -S . -B build-coverage-tests -DBUILD_TESTING=ON -DBUILD_UNIT_TESTS=OFF && ctest --test-dir build-coverage-tests -R test_vulkan_forward_plus_pbr_guards --output-on-failure
• ctest --test-dir build-coverage-tests -R 'test_vulkan_(runtime_regressions|renderer_guards|regression_source_guards|forward_plus_pbr_guards)' --output-on-failure